Posted
by
Zonkon Saturday February 17, 2007 @07:49PM
from the please-bring-your-seatbacks-and-traytables dept.

Maggie McKee writes "The asteroid Apophis has been traversing the void of space for untold years; in just a few decades time it will make a very close pass to Earth, and could make an unwelcome stop on our planet's surface. Even still, it's nothing to get too worked up about. The 20-million-tonne object has a 1 in 45,000 chance of hitting the Pacific Ocean in early April of 2036. If it did hit, it could trigger a tsunami that would do an untold amount of damage to the California coastline and many other places on Earth. Despite the low level of the threat, it's still a real enough danger to prompt the United Nations to develop a protocol about the scenario. We'll get a closeup look at the object in 2029, and at that point we should have a better idea of what 2036 will bring us."

Figuring out the exact speed of an asteroid, relative to us, is apparently a tad easier than figuring out its exact course. According to the data we have, the possible path of the asteriod is a cone.... the earth is inside that cone currently. Earth takes up about 1/45,000th of that cone, specifically. We know when it will get here, if it does get here, with a good degree of accuracy. And we know what direction it would be coming from. So that rules out it landing in, say, Cuba - it would be coming from the wrong direction to hit there at the time of impact.

The PTS relates the impact risk to the background risk in a logarithmic way -- that is, the probability of Apophis hitting us is 0.003 times the probability that we will be struck by some other asteroid of equal or larger size first. Or, put another way, yes we should be worried about asteroid impacts, and yes we should keep watching Apophis, but it's not (by our understanding) a big cause to go and panic.

That said, Apophis is the second highest ranked asteroid we know about by the PTS, behind 2007 CA19 at -0.91 (potential impact in 2012). And if it gets the people with the budgets to start considering the problem, that's a good thing. Right now, though, it would seem that our best use of money is to spend more effort looking for asteroids -- so far, the number we find appears to be fairly well correlated to how hard we look, suggesting that we have found a very, very small fraction of the NEOs out there.

The object 2007 CA19 [nasa.gov] has a better chance (as of right now) of hitting the Earth than 99942 Apophis (2004 MN4) [nasa.gov] does. The former is also about four times larger than the latter and would have more than double the velocity at impact if it were to hit.

Actually, the uncertainty mostly isn't due to error in position; it's due to the fact that, when we observe a NEO, it's a point of light in the sky. We really don't know how far away it is. If it's near to the Earth and Sun, it moves more quickly; if it's farther away, it moves more slowly. If you remember that things move in circles or ellipses around the Sun, then you might get the idea that the uncertainty "ellipse" (due to a small error in position left-right, but a very large error in depth) due to the different orbital velocities, it "stretches out" over time, wrapping the ellipse's major axis around the Sun until it's basically a straight line.

There's a fantastic animation of this process at Spaceguard's [esa.int] site, just scroll down to the second animation.

One of them is to look at history- written history, archaeology, and geology. There are no written accounts, as far as I know, of a meteorite causing significant numbers of human casualties, either through an impact or through a tsunami induced by impact.

Well, the dinosaurs would have left written accounts, but they were all dead.

More seriously, we do have historical record of even minor meteor showers causing casualties, the biggest reportedly in Chiing-yang, China in 1490, in an apparent Tunguska-like event, killing a possible "tens of thousands". Mostly its onesies and twosies, though. Tunguska itself, detonating in the middle of nowhere, Sibera, injured the 20 people who were within 50 km of the blast, and killed two. Thousands of reindeer were killed.

Should Apophis (or something that size) hit Earth, the energy release would be about 10 to 20 times that of the Tunguska or Arizona impacts (those were in the 10-20 megaton range), and about 2 or 3 times that of the Krakatoa explosion. Since 3/4 of the planet is water-covered, odds are that most large impacts hit water and cause damage through the result tsunamis. (And yes, we get a few in the several-kiloton range each year - mostly in the middle of nowhere - as has been documented by surveillance satellites.)

Sure, Apophis is no Dinosaur Killer, but it could cause quite a mess depending on if and where it hits.

They don't know exactly where it will intersect Earth's orbit, but they do know, within an hour or two when it will cross our path. At that time of day (or night, whatever) the Pacific Ocean will be more or less facing the direction the asteroid's coming from. Given the size of the Pacific it's reasonably likely that if the asteroid does hit Earth, it'll be somewhere there.

Scientists may also have an idea of the latitude or longitude it will hit, narrowing the window further.